1. Field of the Invention
This invention relates to novel cathode and electrolyte materials for intermediate temperature solid oxide fuel cells and ion transport membranes based on oxides having perovskite related structures and an ordered arrangement of A site cations.
More specifically, the present invention relates to a family of compositions including at least one compound having the general formula (I):(ABO3)p(A′BO2+x)q(A′O2+x)r  (I)where p, q, and r are integers, x is a real number, A is a divalent metal, A′ is a trivalent metal and B is a metal capable of ready conversion between its plus three oxidation state and its plus four oxidation state or mixtures or combinations of such metals and where the cathode compositions possess both oxygen ion diffusivity and electronic conductivity. Cathode compositions also find applications in ion transport oxygen separation membranes and cathode.
More specifically, the present invention also relates to a family of electrolyte compositions including at least one compound having the general formula:(AB′O3)p(A′B′O2+x)q(A′O2+x)r  (II)where p, q, and r are integers, x is a real number, A is a divalent metal, A′ is a trivalent metal and and B′ is a non transition metal and where the electrolyte compositions is a pure ionic conductor. Electrolyte compositions can be used in chemical sensors.
More specifically, the present invention also relates to fuel cells and separation cells including a composition of formulas (I) and/or (II) and the methods for making and using same.
2. Description of the Related Art
Solid oxide fuel cells (SOFCs) have the promise to improve energy efficiency and to provide society with a clean energy producing technology. The high temperature of operation enables the solid oxide fuel cell to operate well with existing fossil fuels and to be used in combined heat and power applications or efficiently coupled with turbines, to give very high efficiency conversion of fuels to electricity. SOFCs are quiet and non-polluting and their inherent high efficiency leads to lower greenhouse gas emissions.
At present, solid oxide fuel cells operating at 1000° C. utilize a yttria stabilized zirconia electrolyte (YSZ), a lanthanum strontium manganite cathode, and a nickel-YSZ cermet anode. Cells are connected by a lanthanum strontium chromite bipolar plate. Various geometries for the cell design have been investigated, but the most developed is the Siemens-Westinghouse tubular configuration in which the YSZ electrolyte film (30-40 μm) is supported on a 1.5 m long tube of porous lanthanum strontium manganite. The Siemens-Westinghouse design has been demonstrated successfully at the 100 kW scale.
While the technology has been successfully established, costs remain too high to permit widespread introduction of SOFCs into the marketplace. Cost reduction requires both improvements in the properties of the materials, particularly the electrodes, and the development of inexpensive fabrication processes. Lowering the operating temperature has a significant impact on cost by allowing the use of less expensive materials in interconnects and heat exchangers. Lower temperatures also lead to an increase in the reliability of SOFC systems by reducing problems associated with thermal cycling and performance degradation due to inter-diffusion or reaction of the individual components. Intermediate temperature SOFCs in both planar and tubular configurations at the 3-10 kW scale, for distributed combined heat and power and for auxiliary power are currently being developed in the U.S. by several organizations.
Operation of SOFCs at intermediate temperatures (500-800° C.) requires new combinations of electrolyte and electrode materials that will provide both rapid ion transport across the electrolyte and electrode-electrolyte interfaces and efficient electrocatalysis of the oxygen reduction and fuel oxidation reactions.
Mixed ionic electronic conducting (MIEC) oxides are the major functional component of ion transport membranes (ITMs). ITMs operate at high temperature by catalyzing the dissociation and reduction of an oxygen molecule at one membrane surface followed by coupled transport of an oxygen ion and two electron holes through the bulk material. On the second surface, the oxide ion acts as an oxidant for the catalyzed reaction of hydrocarbons such as methane to form synthesis gas or recombines to give molecular oxygen releasing electrons back into the membrane. In the latter case, the membrane functions as a high temperature oxygen separation device. Ion transport membrane systems are simpler in design than SOFCs in that no external circuit is required, but at the same time the materials requirements are very demanding because of the large gradient in oxygen activity across the membrane. The overall performance of an ITM is determined by the bulk transport and surface reactions which are strongly coupled together.
Thus, there is a need in the art for a new class of materials that will allow SOFCs and ITMs to operate at intermediate temperatures.